Process for degassing an aqueous plating solution

ABSTRACT

A process for removing oxygen from a copper plating solution is provided. The solution is passed through a degasser comprising a shell and hollow hydrophobic fiber porous membranes wherein the shell while a vacuum is drawn on the surfaces of the fibers opposite the fiber surfaces contacted by the solution. Gas passed through the fiber walls while liquid is prevented from infiltrating the fiber pores. The composition of the solution is monitored so that the composition can be retained substantially constant by adding components of the solution as needed.

This application claims the benefit of Provisional Application No.60/267,295, filed Feb. 7, 2001.

FIELD OF THE INVENTION

This invention relates to a process for removing all dissolved gassesfrom aqueous electroplating and electroless plating bath solutions. Moreparticularly, this invention relates to a process for removing dissolvedgasses including oxygen from aqueous copper plating and electrolessplating bath solutions.

BACKGROUND OF THE INVETION

Recently copper electrochemical deposition processes have becomeavailable to form electrically conductive pathways on semiconductorchips. Copper electrochemical deposition process for the damascenestructures of high aspect ratios for semiconductor manufacturing is anew application of the conventional electroplating process. Theelectroplating of high aspect ratio devices involves the use of copperplating to fill high aspect ratio sub-micron trenches and viaspositioned on semiconductor chips. An acidic copper sulfate solution ofoptimized composition has proven to be the best formula for platingmicro-features. Typically, the process consists of circulating a platingsolution from a reservoir to a plating cell and back to the reservoir. Acopper anode in the plating cell provides the source of copper, which isdeposited on the cathode comprising a silicon wafer with the damascenestructure.

The final performance of the plated wafer depends on the electrical andmorphological properties of the deposited copper film. Theelectrochemical bath composition plays a significant role in depositedcopper film properties. The solution concentration of copper and sulfateions, chloride ions, metallic impurities, and organic additives all areimportant parameters for providing acceptable copper deposition.

The organic additives added to the bath include accelerators,brighteners, suppressors, and levelers. The combination of theseadditives determines fling properties as well as the film's initialgrain size, brightness or roughness. The optimum bath composition ismaintained by periodic analysis and replenishing of the plating bath.

During operation of the bath, the solution is constantly exposed toenvironmental oxygen as the surrounding air is entrained into therecirculating plating solution. It has been determined that some of theorganic additives are sensitive to oxidative decomposition. Acceleratedorganic additive consumption changes the chemical composition of thebath which, in turn, can adversely affect the acceptability of thedeposited copper film. The bath chemical composition can be changed bothby the depletion of one or more organic additives and by the increasedconcentration of organic decomposition produced.

The presence of dissolved gas such as oxygen in the plating bath alsocan cause the formation of undesirable microvoids in the plated copperfilm This, in turn, can cause reduced electrical conductivity in thecopper pathways formed in the semiconductor surface.

Accordingly, it would be desirable to provide a copper electrochemicaldeposition process wherein decomposition of organic additives in acopper plating bath is controlled and minimized. In addition, it wouldbe desirable to provide such a process wherein dissolved gas in a copperplating bath is removed.

SUMMARY OF THE INVENTION

The present invention is based upon the discovery that oxygen can beremoved from an aqueous copper plating bath containing organic additivesto stabilize the bath against decomposition of the organic additives bypassing the bath through a degasser apparatus comprising a shell(housing) having hydrophobic hollow porous membranes (fibers) whichextend through the shell. The hollow hydrophobic porous membrane permitspassage of gas therethrough while preventing passage of liquidtherethrough. The plating bath solution can be passed either through theshell to contact the outer surfaces of the hollow hydrophobic porousmembrane or through the lumens of a hydrophobic hollow porous membranesunder conditions that prevent significant intrusion of the bath solutioninto the membrane pores while permitting passage of oxygen gas throughthe pores. The degasser wherein the bath solution is introduced into theshell to contact the outer surfaces of the hollow membrane is referredto in the art as a “shell side degasser”.

In accordance with this invention, a copper anode and a cathodecomprising a substrate such as a silicon wafer, upon which anelectrically conductive copper pathway is to be plated, are immersed inan acidic aqueous copper plating bath in a plating step. The platingbath contains organic additives which facilitate plating of copperincluding accelerators, brighteners, suppressors and levelers. Aqueouscopper plating solution is directed to the plating step by being passedthrough a filter to remove particles therein and then through the hollowfiber membrane degasser to remove dissolyed oxygen from the solution.Degassing is effected with the hollow fiber membranes under conditionsthat intrusion of liquid through the pores of the membranes isprevented. The plating solution is removed from the plating bath and isdirected to a reservoir for the solution where its composition can bemonitored to determine whether additional organic additive or acidshould be added thereto in order to maintain the desired compositionwhich is efficient for attaining satisfactory copper plating in theplating step.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic flow sheet illustrating the process of thisinvention.

FIG. 2 is a graph showing consumption of organic additives in a copperplating bath without removing oxygen in Example 1.

FIG. 3 is a graph showing consumption of two organic additives withoxygen removal in Example 1.

FIG. 4 is a graph showing consumption of an organic additive with andwithout oxygen removal in Example 1.

FIG. 5 is a graph showing consumption of an oxygen additive utilizingthe degassing steps in parallel in Example 1.

FIG. 6 is a graph of efficiency in removing gas in the degasser ofExample 4.

FIG. 7 is a graph showing consumption of additives in the degasser ofExample 4.

DESCRIPTION OF SPECIFIC EMBODIMENTS

Degassing of an aqueous acidic copper plating solution to remove oxygenis effected by passing the solution through a degasser comprising ashell through which extend hydrophobic hollow porous membranes. Theplating solution can be passed through the shell to contact the outersurfaces of the hollow porous fiber membranes or through the lumens ofthe hollow porous fiber membranes. The solution is passed through thedegasser under conditions to prevent liquid flow through the pores ofthe membranes while permitting gas flow through the membrane pores.Thus, the membrane surfaces are not wet by the solution therebypreventing significant liquid intrusion into the membrane pores. Whilethe solution is passed either through the shell or through the hollowporous fiber membranes, a subatmospheric pressure is effected on thesurfaces of the membranes opposite the membrane surfaces contacted bythe bath by removing gas either from the lumens of the membrane or fromthe housing.

The hollow porous fiber membranes are formed from a hydrophobic polymerhaving a surface energy equal to or greater than about 23 dynes/cm,preferably greater than about 25 dynes/cm. Representative suitablehydrophobic polymers include skinned hydrophobic polymers such asperfluoroalkoxy polymers (PFA) such as perfluoro (alkoxy vinylether),fluorinated ethylene-propylene polymer (Teflon FEP) or the like. Themembranes typically have a bubble point greater than about 100 psi.Suitable skinned membranes can be produced by the process of U.S. patentapplication No. 60/117,854, filed Jan. 29, 1999 which is incorporatedherein by reference.

The vacuum utilized to effect degassing to remove oxygen from thesolution positioned either within the shell or within the lumens of thehollow porous fiber membranes is between about 10 inch Hg and about 29inch Hg, preferably between about 25 inch Hg and about 28 inch Hg.

Typically, the fibers range in length between about 8 inches and about20 inches although fibers of shorter or longer length can be employed.Typical conditions of flow of the aqueous solution through the shell orthrough the fibers are between about 10 and about 30 liters/minute.Under these process conditions, oxygen concentration in the solution isreduced to below about 6 ppm, preferably below about 3 ppm.

The degassing apparatus of the invention generally is made by pottingthe hollow fiber porous membrane to both ends of a shell (housing) sothat liquid flow through the degasser is effected through the lumens ofthe hollow fibers or through the portion of the shell interior notoccupied by the hollow fibers. Potting is a process of forming a tubesheet having liquid tight seals around each fiber. The tube sheet or potseparates the interior of the final contactor from the environment. Thepot is thermally bonded to the housing vessel to produce a unitary endstructure. The unitary end structure comprises the portion of the fiberbundle which is encompassed in a potted end, the pot and the end portionof the hydrophobic thermoplastic housing, the inner surface of which iscongruent with the pot and bonded to it. By forming a unitary structure,a more robust degasser is produced. That is, it is less likely to leakor otherwise fail at the interface of the pot and the housing. Asuitable potting and bonding process is described in U.S. patentapplication No. 60/117,853 filed Jan. 29, 1999, the disclosure of whichis incorporated by reference.

Potting and bonding are done in a single step. An external heating blockis used for potting one end at a time. The perfluorinated thermoplasticend seals are preferably made of poly (tetrafluoroethylene-co-perfluoro(alkylvinylether)) having a melting point of from about 250° C. to about260° C. A preferred potting material is Hyflon® 940 AX resin, fromAusimont USA Inc. Thorofire, N.J. Low viscosity poly(tetrafluoroethylene-cohexafluoropropylene) with low end-of-melttemperatures as described in U.S. Pat. No. 5,266,639 is also suitable.The process involves heating the potting material in a heating cup ataround 275° C. until the melt turns clear and is free of trappedbubbles. A recess is made in the molten pool of potting material thatremains as a recess for a time sufficient to position and fix the fiberbundle and housing in place. Subsequently, the recess will fill with themolten thermoplastic in a gravity driven flow.

A unitary end structure, by which is meant that the fibers and the potare bonded to the housing to form a single entity consisting, forexample, of perfluorinated thermoplastic materials is prepared by firstpretreating the surfaces of both ends of the housing before the pottingand bonding step. This is accomplished by melt-bonding the pottingmaterial to the housing. The internal surfaces on both ends of thehousing are heated close to its melting point or just at the meltingpoint and immediately immersed into a cup containing powdered (poly(PTFE-CO-PFVAE)) potting resin. Since the surface temperature of thehousing is higher than the melting point of the potting resins, thepotting resin is then fused to the housing resin—a condition for bondingto occur. The housing is then taken out and polished with a heat gun tofuse any excess unmelted powder. Without this pretreatment step, thehousing surfaces often detach from the potting surfaces because ofabsence of intermixing of the two resins.

The unitary end structure(s) is cut and the lumen of the fibers exposed.The potted surfaces are then polished further using a heat gun to meltaway any smeared or rough potted surfaces. A solder gun can be used tolocally remelt and repair any defective spot, sometimes with the help ofa drop of melted resin.

The process of this invention is illustrated in FIG. 1. As shown in FIG.1, a plating bath 10 is provided which includes a housing 12, an innertube 14 which includes a copper anode 16 and a cathode substrate to beplated such as a silicon wafer 18. The surface of the solution inhousing 12 can be blanketed with nitrogen or an inert gas such as argon,helium or the like to reduce oxygen dissolved in the solution. Degassedaqueous acidic copper solution containing organic additives is directedthrough conduit 20 into inner tank 14 wherein a voltage is establishedbetween anode 16 and cathode 18. Spent solution is removed from tank 14as indicated by arrows 22 and 24 through conduit 26 and is directed toreservoir 28. At reservoir 28, the spent solution 30 can be analyzed fororganic concentration and concentration of additive decompositionproduct. Based on the analyses, organic additives can optionally beadded to solution 30. The solution 30 then is pumped by pump 32 throughparticle filter 40, conduit 33 and then through degasser 42 containingthe hollow porous fiber membranes in a housing as described above andwherein a vacuum is drawn through conduit 44. The degassed solution withreduced oxygen concentration is returned to tank 12 through conduit 20.It is to be understood that a plurality of degassing units 42 can beutilized either in parallel or in series to reduce oxygen content of thesolution being cycled through the process of this invention.

The following examples illustrate the present invention.

Two types of experiments were performed: [1] without degasser and [2]with degasser in the plating bath system to determine if the additiveconsumption can be controlled/reduced.

EXAMPLE 1 Experiments Without Degasser

Experiments were performed in a copper electroplating tool. The platingsolution from a reservoir (˜75 liters) is circulated (˜17 liters/minflow rate) through a plating cell containing a silicon wafer cathode anda copper anode. The solution additives are maintained at adequate levelby periodically analyzing the bath for composition and adding the makeup amount.

An analysis of two key additive components and dissolved oxygen in thesolution, for one week, is profiled in FIG. 2 wherein X and Y are twodifferent organic additives. FIG. 2 plots Amp. Hours vs. additiveconcentration or oxygen concentration. As shown in FIG. 2, the X and Yadditives were consumed in the presence of oxygen.

EXAMPLE 2 Use of Single Degasser

A second set of experiments were conducted as described in Example 1 butwith a degasser unit turned on (˜26 Hg vacuum). The degasser unitincluded 10 inch hollow fiber skinned PFA ultrafiltration membranes. Thedissolved oxygen and additive concentration in the bath were monitoredas profiled in FIG. 3.

As shown in FIG. 3, the process with a degasser lowers the dissolvedoxygen in the solution by about 1 ppm. The concentration of component Xin the additive is less affected (consumed) with the degasser or withoutthe degasser. These results are shown in FIG. 4. The data show that withthe degasser on, the consumption of the additive component X was less.

EXAMPLE 3 Three Degassers and Nitrogen Blanket

Three degasser modules of the type used in Example 2 were installed (ina parallel configuration) in the copper plating unit. The objective wasto determine incremental improvements in degassing efficiency and itseffect on the additive consumption, over time.

The system performance was also improved by reducing/eliminating thesources of oxygen entrainment into the plating solution at the celloverflow, drain pipe return line, and the solution reservoir by infusingnitrogen and covering these areas with appropriate plastic lids orplastic sheeting.

Preliminary results indicate the degassing efficiency increased to about40% with three degasser (vs 10–15% with one degasser). Uponadding/covering various exposed areas with a nitrogen blanket, there wasa significant improvement in the degassing efficiency in ˜50%. The bathsamples were analyzed for additive consumption. The results show adramatic decline in the additive consumption under the high degassingcondition (dissolved oxygen in 4–5 ppm range) (See FIG. 5).

It is seen from the tests that a reduction in dissolved oxygen using thedegasser, inline has the benefit of lowering the consumption of someadditives in a copper electroplating bath

EXAMPLE 4

This example illustrates the process of this invention utilizing a shellside degasser wherein a plating solution contacts the outer surfaces ofhollow hydrophobic fiber membranes positioned within a shell.

A Liqui-cell degasser available from Celgard, Inc., Charlotte, N.C.,U.S.A., (liquid flow on the outside of hollow fiber, vacuum on the lumenside) was installed and operated for about 10 days. The integrity of thedegasser was very good. There was no sign of weeping or leaking. Thesingle pass efficiency was 37+/−8% at 4.5 GPM solution flow rate. Thetotal system efficiency was about 73+/−5%, which was calculated based onthe saturated O₂ level in the bath. The analysis of additives showedthat the degasser reduced the consumption rate of Additive X. (A) Theintegrity is determined in two ways. [1] Before the installation, thedegasser is subjected to a 60 psi water pressure on the shell side. Anystructural defects would manifest by leaks at the potting ends. Anabsence of any such leaks would indicate the degasser is integral. [2]After the installation, the test involves a visual observation for thepresence of any plating solution on the gas side. (B) Total systemefficiency. The system efficiency for oxygen removal is the ratio of thedissolved oxygen concentration in the bath at any time to the initialoxygen level at the start of the run.${\%\mspace{14mu}{system}\mspace{14mu}{efficiency}} = \frac{{bath}\mspace{14mu}{oxygen}\mspace{14mu}{concentration}\mspace{14mu}{at}\mspace{14mu}{time}\mspace{14mu} t\mspace{14mu}({ppm})}{{initial}\mspace{14mu}{bath}\mspace{14mu}{oxygen}\mspace{14mu}{concentration}\mspace{14mu}({ppm})}$

Experimental

The experiments were performed in a re-circulated copper plating toolunder the following operating conditions:

-   -   A used Gen6B2 Anode package with about 8,000 {umlaut over        ({hacek over (u)}m plated    -   Anode flow rate: 340 ml/min without the anode downstream filter    -   40 ma/cm² current density with rotating cathode at 20 rpm.    -   Flow rate=4.5+/−0.3 GPM, Temperature=15+/−2.0° C., Additive        X=5.0+/−1.0 ml/L, Y=14+/−2.0 ml/L, Cl=60+/−10 ppm and        H₂SO₄=20+/−10 g/l    -   24 hours operation without interruptions

Results

Degasser Efficiency

As shown in FIG. 6, the degasser single pass efficiency was 37+/−8%throughout the testing period. The total system efficiency was about73+/−5%, which was calculated based on the saturated O₂ level in thebath.

Additive Consumption Results

Additive consumption rate was measured with and without degasser. Asshown in FIG. 7, the degasser reduced the consumption rate of additive“X” by nearly 50%; degassing had less effect on the consumption rate ofadditive “Y”. Based on the normal consumption rate of 0.15 ml/Amp. hrs(as circular points shown) for Gen6b3, degasser reduced the consumptionrate by 38%.

1. A process for reducing consumption of at least one organic additivein a copper plating bath which comprises: passing said copper platingbath from a housing containing said copper plating bath, said housingcontaining an anode and a cathode comprising a substrate to be platedwith copper, to a degasser containing hollow porous fiber membraneshaving a hydrophobic surface; drawing a vacuum in lumens of said hollowporous fiber membranes to remove oxygen from said copper plating bathand passing degassed copper plating bath to said housing.
 2. The processof claim 1 which includes the step of removing particles from saidcopper plating bath positioned between said housing and said degasser.3. The process of any one of claim 1 or 2 which includes the step ofadding an organic plating additive to said copper plating bath betweensaid housing and said degasser.
 4. A system for plating a substrate withcopper which comprises: a housing containing an anode, a cathodecomprising said substrate and a copper plating bath containing at leastone organic additive; a degasser unit containing hollow porous fibermembranes having a hydrophobic surface, means for drawing a vacuum inlumens of said hollow porous fiber membranes; and means for circulatingsaid copper plating bath between said housing and said degasser unit. 5.The system of claim 4 which includes means for removing particles fromsaid copper plating bath, said means positioned between said housing andsaid degasser unit.
 6. The system of anyone of claim 4 or 5 includingmeans for adding said at least one organic additive to said copperplating bath.
 7. The system of claim 6 wherein an atmosphere within saidhousing comprises a gas selected from the group consisting of nitrogenand an inert gas.
 8. The system of any one of claim 4 or 5 wherein anatmosphere within said housing comprises a gas selected from the groupconsisting of nitrogen and an inert gas.